1. Field of the Invention
The present invention relates to an input protection circuit for protecting internal circuitry of a handheld electric device. More specifically, the present invention discloses an input protection circuit with low power consumption.
2. Description of the Prior Art
In modern society, people hope to receive all kinds of information via handheld electric devices, such as a personal digital assistant (PDA), a mobile phone, etc. These handheld electric devices have complex internal circuitry for dealing with great amount of data. The internal circuitry must have a certain threshold voltage to provide a direct current (DC) bias voltage for operating regularly. When a DC voltage exceeds the threshold voltage or a reverse DC voltage inputs to the internal circuitry, the internal circuitry will be damaged and the handheld electric device will have abnormal operation.
For preventing improper damages of the internal circuitry, the handheld electric device has an input protection circuit for protecting the internal circuitry. Please refer to FIG. 1 of a diagram of an input protection circuit 12 applied in a handheld electric device 10 according to the prior art. The handheld electric device 10 has a direct current (DC) power supply 24 for providing DC power. The DC power supply 24 inputs the DC power to an internal circuitry 14 through the input protection circuit 12 of the handheld electric device 10 for protecting the internal circuitry 14 of the handheld electric device 10. The input protection circuit 12, electrically connected between the DC power supply 24 and the internal circuitry 14, has a power socket 16. The power socket 16 comprises a positive input node 16A and a ground node 16B, respectively electrically connected to two output nodes 24A and 24B of the DC power supply 24 for inputting the DC power provided from the DC power supply 24 to the input protection circuit 12. The input protection circuit 12 further comprises two power connection channels 21 and 23, respectively electrically connected to the positive input node 16A and the ground node 16B of the power socket 16 and the positive input node 14A and the ground node 14B of the internal circuitry 14 for forming electric connection between the DC power supply 24 and the internal circuitry 14.
The prior art input protection circuit 12 further comprises a power diode D1 and a pnp-type bipolar junction transistor (BJT) Q1 respectively connected to the power connection channel 21 for controlling the DC power inputting through the power connection channel 21 to the internal circuitry 14 so as to protect the internal circuitry 14. A base of the pnp-type BJT Q1 electrically connects to an npn-type BJT Q2 functioning as a control transistor, and a base of the npn-type BJT Q2 electrically connects to an output node 20A of an overvoltage protective circuit 20. The overvoltage protective circuit 20 further comprises two input nodes 20B and 20C respectively connected to two power connection channels 21 and 23.
An operation principle of the prior art input protection circuit 12 can be described as follows. A BJT can utilize a base current to turn the current between a collector and an emitter on and off. When a DC voltage provided by the DC power supply 24 is below a threshold voltage of the internal circuitry 14, the power diode D1, the transistor Q1, and the control transistor Q2 are all turned on so that the current provided by the DC power supply 24 flows into the internal circuitry 14 through the power connection channel 21. When a DC voltage provided by the DC power supply 24 exceeds the threshold voltage of the internal circuitry 14, the overvoltage protective circuit 20 with the two input nodes 20B and 20C electrically connecting between the power connection channel 21 and 23 will detect a overvoltage between the positive node 16A and the ground node 16B. Then, the overvoltage protective circuit 20 draws the base current out of the control transistor Q2 through the output node 20A so as to reduce the on current flux between the collector and the emitter of the control transistor Q2, and even turning off the control transistor Q2. Current reduction or elimination between the collector and the emitter of the control transistor Q2 causes the base current of the transistor Q1 to be reduced or turned off thereby causing the current between the emitter and the collector of the transistor Q1 to be reduced or turned off. This prevents the DC current caused by exceeding the threshold voltage from flowing from the DC power supply 24 into the internal circuitry 14 through the transistor Q1.
If the output node 24A of the DC power supply 24 which will connect with the positive input node 16A is connected to the ground node 16B, and if the output node 24B of the DC power supply 24 which will connect with the ground node 16B is connected to the ground node 16A, the power diode D1 positioned at the power connection channel 21 will be turned off so as to protect the internal circuitry 14.
A defect of the prior art input protection circuit 12 is the utilization of the BJT to control the transistor Q2. When the voltage of the DC power supply 24 is below the threshold voltage, the control transistor Q2 stays on so as to turn on the transistor Q1. Thus, the DC power of the DC power supply 24 is electrically connected to the internal circuitry 14. The overvoltage protective circuit 20 outputs a current from the output node 20A to the base of the control transistor Q2 because the BJT must use the base current to control the transistor Q2. As a result, the overvoltage protective circuit 20 must consume power to maintain normal operation. Volume of the prior art overvoltage protective circuit 20 cannot be lessened because the overvoltage protective circuit 20 consumes power for preventing the prior art input protection circuitry 12 from overheating. Furthermore, the power diode D1 positioned at the power connection channel 21 must admit the DC power to pass from the DC power supply 24 to the internal circuitry 14, meaning that the power diode D1 must be a diode with a large volume and the ability to bear high DC power. The above-mentioned reasons increase power consumption of the prior art input protection circuit 12 and do not allow the whole volume of the input protection circuit 12 to be lowered.
It is therefore a primary objective of the claimed invention to provide an input protection circuit with low power consumption to effectively lessen a volume of the input protection circuit.
The claimed invention discloses an input protection circuit for protecting internal circuitry of a handheld electric device. The internal circuitry has a positive input node and a ground node. The input protection circuit comprises a power socket having a positive input node and a ground node, a bipolar junction transistor (BJT), a metal-oxide semiconductor (MOS) transistor for controlling the on and off states of the BJT, and an overvoltage protective circuit. The power socket is used to electrically connect with two output nodes of a direct current (DC) power supply whose ground node is electrically connected to the ground node of the internal circuitry. An emitter of the BJT is electrically connected to the positive input node of the power socket, and a collector is electrically connected to the positive input node of the internal circuitry. When a reverse DC voltage or a DC voltage exceeding a threshold inputs from the positive input node and the ground node of the power socket, the overvoltage protective circuit will turn off the MOS transistor thereby turning off the BJT to prevent damages of the internal circuitry. When a DC voltage below the threshold inputs from the positive input node and the ground node of the power socket, the overvoltage protective circuit will turn on the MOS transistor thereby turning on the BJT so as to input the DC voltage to the internal circuitry through the BJT.
It is an advantage of the claimed invention that the handheld electric device has an input protection circuit having a lessened volume to effectively prevent a DC voltage from exceeding the threshold voltage and a reverse voltage from damaging the internal circuitry of the handheld electric device.
These and other objectives and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.